Letter to the Editor

IBIS Veto System^*

Background rejection, instrument dead time and zoning performance

¹ Istituto di Astrofisica Spaziale e Fisica Cosmica, Italy
² Departement of Physics and Astronomy, Southampton University, Southampton, UK
³ Universitet I Bergen, Fysisk Institutt, Bergen, Norway
⁴ VEGA IT GmbH c/o European Space Operation Centre-ESA, Darmstadt Germany
⁵ Centrum Bada Kosmicznych PAN (PAS-SRC), Warszava, Poland

Corresponding author: E. M. Quadrini, mauro@mi.iasf.cnr.it

Received: 14 July 2003
Accepted: 8 August 2003

Abstract

IBIS is the high energy imager on board the INTEGRAL satellite. The gamma-ray instruments on board will take advantage of the long uninterrupted observation made possible by the very eccentric orbit (10 000  perigee and 152 000  apogee). A disadvantage for orbits outside the protection of the van Allen belts is the exposure to cosmic and solar particles. Conversely, the background is quite stable throughout the 3 days orbit. In order to maximise the scientific returns and take full use of these almost 3 days continuous observations, IBIS is equipped with a light, very effective Veto System. This ensures a substantial reduction of the background due to the induced photon and hadronic component, in turn enhancing the detector sensitivity. The performance of the IBIS veto as evaluated during telescope commissioning is reviewed. In particular, the efficiency of background rejection and the resulting IBIS dead time are evaluated as well as the impact of different zoning configurations. Measured over the whole energy range, the veto system provides a background suppression effect of ~50% for ISGRI and ~40% for PICsIT. The definitive veto settings optimised for the operational working temperature and background conditions are described.